Large thunderstorms are one of the most damaging of weather phenomena. Hail can devastate crops, flash flooding can inundate towns and homes, lightning can threaten people and ignite fires, and strong gusts can damage transport and infrastructure. Convective storms and associated phenomena cause 5-8 billion euro per year in damage across Europe. Such storms have the potential to be forecast and the public warned beforehand, but forecasting becomes increasingly difficult as the length of a forecast increases. In the near-term, observations and high-resolution computer modelling can provide adequate warning of impending storms, but for periods longer than three days ahead the outbreak of thunderstorms has to be deduced indirectly from the computer forecast even if the large-scale flow is well forecasted.

The aim of this project was to improve our understanding of the relationship between thunderstorms (also called convective storms) and the larger-scale environment in the atmosphere, to provide better understanding of the physical processes responsible to aid forecasters in interpreting the model predictions. Convective storms require three ingredients: sufficient moisture to condense and fuel the storm, instability or the rate at which temperature decreases with height (temperature dropping quickly with height is better), and something to lift air to release the instability.

This project focused on the instability ingredient. In the United States, environments with large instability are believed to occur because of heating over the elevated terrain of the western United States, resulting in the elevated mixed-layer (EML). In Europe, EMLs are attributed to passage over the elevated terrain of central Spain, resulting in the Spanish plume. Such sensible heating of lower-tropospheric air (3-5 km above sea level) by an elevated heat source such as the Rockies or Spanish plateau is a natural explanation for the steep lapse rates in the EML. How much of a contribution is the elevated heating to the formation of instability? The smaller scale of the Spanish high terrain compared to the Rocky Mountains makes it difficult to imagine that the Spanish high terrain creates such large instability.

One hypothesis for the origin of the steep lapse rates is the Sahara Desert, where a well-mixed boundary layer forms steep lapse rates that can be advected away from northern Africa (known as the Saharan Air Layer). Yet, this hypothesis has not been tested, either for the Spanish plume or other regions downstream of high heated terrain. A different factor said to explain the occurrence of instability is the differential transport of air with low temperature or low moisture aloft. Although such explanations have been used in the literature, other studies have questioned the applicability of this factor. The project research asked what processes produce the environment for midlatitude convective storms around the globe. What environments are favourable for instability, and how does this differ around the globe? What are the physical processes that create instability? Is instability - in Europe generally and the UK specifically - attributed to elevated heating, as in the EML of the central United States or by long-range transport? Despite conventional wisdom stating that the elevated mixed layer is responsible for creating the instability downstream of high terrain, it remains untested.

The project aim was to develop a better understanding of the relationship between high terrain, large-scale processes, and instability for midlatitude convective storms. These concerns motivate a multifaceted research project to answer these questions. Q1: What are the physical processes responsible for creating instability? Q2: How does topography create a favourable environment for deep moist convection? Q3: How important is differential temperature and moisture advection to creating insta

Objectives

Questions about instability and the synoptic-scale environment of midlatitude convective storms motivated the project, the purpose of which was to challenge conventional wisdom about the origin of the environments favourable for convective storms.
Specifically, its objectives were to test three hypotheses that challenge conventional wisdom.

H1: Layers with high lapse rates primarily originate from synoptic-scale processes remote from heated terrain that destabilise the layer, thus questioning the validity of the elevated mixed layer concept for producing steep lapse rates in all episodes of high lapse rates.

H2: Specifically, the origin of the high lapse rates in the Spanish plume is not an elevated mixed layer over the Spanish plateau, but a result of long-distance advection from the Sahara Desert, synoptic-scale processes destabilising the layer, or both.

H3: Rapid local destabilisation is not a result of differential advection of temperature or moisture with height.

Other processes in association with short-wave troughs create a favourable environment for convective storms. By challenging conventional wisdom through these three testable hypotheses, the project participants aimed to transform our understanding of convection. Quantitative evaluation of these hypotheses has not occurred largely because of the lack of the tools or the insight to test them before the project. With reanalysis and mesoscale modelling commonplace at the time of the project's inception, its research to test these three hypotheses became timely. In addition, the project participants believed that the conventional wisdom the sought to address would be sufficiently novel as to necessitate rewriting textbooks on convective storm environments. The project participants aimed to test these three hypotheses by understanding the spatial and temporal variability of high lapse rate environments and how different physical processes control this variability.

The research consisted of four work packages (WPs) that explore these environments ranging from the midlatitudes across the globe to local environments within Europe, including the UK. In WP1, a climatology was constructed of convective parameters such as CAPE and lapse rate using observed soundings and reanalysis. In WP2, the regional variability of unstable episodes was explored through clustering and synoptic compositing. WP3 explored the importance of elevated heating in producing high lapse rates through real-data, quasi-idealised and idealised simulations from a cloud-resolving model, coupled with diagnostics such as trajectories and static stability tendency equations. WP4 focused on synthesis and outreach, producing a synthesis article and a national meeting on convective storms for the general meteorological community. Forecaster interaction and public engagement occurred throughout the project.